Method for coating a component and coated component

ABSTRACT

A surface to be coated of a component is roughened by a laser. The roughened surface is coated by cold spraying. A component may be produced according to the method, which is coated with a layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage entry under 35 USC § 371 of PCTInternational Application Number PCT/EP2019/072137, filed Aug. 19, 2019,and claims priority to

German Publication No. 10 2018 215 389, filed Sep. 11, 2018, the entiredisclosures of each of which is expressly incorporated by referenceherein.

FIELD OF THE DISCLOSURE

The present disclosure relates to a method for coating a component. Thepresent disclosure further relates to a component coated in accordancewith the method.

BACKGROUND

Some components may be coated by cold spraying. A surface of thecomponent may be roughened by sandblasting in order to improve theadhesion between a substrate consisting of steel and the coatingmaterial to be applied. Tilted structural elements may be provided on asurface in order to be able to coat regions of a component that aredifficult to access satisfactorily by cold spraying.

In cold spraying (cold gas spraying), a surface of a component is coatedby depositing a coating material in powder form. A heated gas such asnitrogen or helium is accelerated to very high velocities, for exampleby expansion in a de Laval nozzle, in the direction of the surface to becoated. The velocities are above the speed of sound and amount to 500 to1000 m/s, for example. The powdered coating material is injected intothe accelerated gas jet. This causes the powdered coating material toimpact the surface to be coated at such a high velocity that theparticles of the powder plastically deform on impact and as a resultadhere to the surface to be coated. In contrast to thermal coatingprocesses such as plasma spraying, arc spraying, flame spraying, theparticles of the powder are not melted.

By the impact of a powder particle on the surface to be coated a shearstress is generated, which plastically deforms the material of a powderparticle. The resulting shear stress leads to an adiabatic shearinstability, which brings surface areas of a powder particle totemperatures close to the melting point of the coating material and thussoftens them, thus bonding causing powder particles and component toeach other. A good adhesion between a substrate and the coating materialin cold spraying therefore requires that sufficient shear instabilitiesare generated by the impact.

The roughness of a surface to be coated may influence an adiabatic shearinstability upon impact.

SUMMARY

One aspect of the present disclosure includes a method for coating acomponent, in which a surface to be coated of the component is roughenedby a laser and the roughened surface is coated by a thermal sprayingprocess with high energy, such as cold spraying. In some embodiments,roughening is done in a crater-shaped manner. The component can becoated with metals, polymers, ceramics, composite materials andnanocrystalline powder by cold spraying. In order to achieve sufficientadiabatic shear instabilities on impact and thus good adhesion,sufficiently smooth surfaces may be used.

The present disclosure is explained in more detail below with usingexamples and figures.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 shows two sectional views of a surface generated according to thepresent disclosure;

FIG. 2 is a top view of surface to be coated of a component;

FIG. 3 is a transverse section of a component coated according to thepresent disclosure.

DETAILED DESCRIPTION

According to one embodiment of the present disclosure, a surface to becoated of a component is ground and/or polished, i.e. smoothed, beforeit is roughened by means of a laser. In this embodiment, ground and/orpolished surface areas may remain even after processing by a laser inorder to favorably influence adiabatic shear instabilities and therebyachieve further improved adhesion properties.

Grinding may be performed with a grinding machine and polishing can beperformed with a polishing machine. Polishing may be performed using apolishing paste or a suspension, each with polishing grains includedtherein.

In one embodiment of the present disclosure, the surface to be coated ofthe component is exposed by laser at predetermined locations in such away that crater-shaped depressions are produced at the exposed locationsand other areas of the surface to be coated remain in the previous stateeven after the production of crater-shaped depressions, i.e. inparticular in a ground and/or polished and thus smoothed state. In thisway, the ratio between crater-shaped depressions and areas not roughenedby laser can be optimized in order to achieve particularly good adhesionproperties.

In one embodiment of the present disclosure, the surface to be coated ofthe component is repeatedly exposed by laser in pulses at thepredetermined locations in such a way that crater-shaped depressions areformed at the exposed locations, in particular to achieve crater depthsof at least 10 μm or 20 μm, or to achieve crater depths of at least 30μm, in order to further improve the adhesion properties. It has beenfound that too great depths again worsen adhesion properties. Therefore,the depth of a crater-shaped depression may be limited to 60 μm, or to50 μm, in some embodiments.

In order for a crater-shaped depression to contribute particularly wellto stable adhesion, the diameter of the crater-shaped depression at theupper edge corresponds approximately to the depth of the crater-shapeddepression. Thus, if a crater-shaped depression at the upper edge has adiameter of about 40 μm, for example, then a depth of about 40 μm isincluded. In some embodiments, the depth does not deviate more than 20%from the diameter. Thus, if the diameter is 40 μm, the depth should beat least 32 μm and no more than 48 μm to further improve adhesionproperties. In one embodiment, the crater-shaped depressions have adiameter of 30 μm to 50 μm at the upper edge and a depth of 30 μm to 50μm.

Time intervals are provided between the individual light pulses in sucha way that material melted by laser light can first solidify againbefore the predetermined location is again exposed. Crater-shapeddepressions can thus be optimized in a further improved manner. Theadhesion between the component and the coating can thus be furtherimproved.

Each predetermined location is exposed by the light of the laser atleast 3 times, at least 5 times, and/or not more than 25 times, and/ornot more than 15 times, in order to produce crater-shaped depressions inan optimized manner, in order to thus achieve particularly good adhesionproperties.

In one embodiment of the present disclosure, the crater-shapeddepressions have circular diameters. By this embodiment it is achievedthat areas of the surface to be coated remain between crater-shapeddepressions that have not been exposed by the laser and therefore havenot been roughened.

In one embodiment of the present disclosure, the upper edge of thecrater-shaped depressions protrudes in a bead-like manner with respectto areas of the surface to be coated that have not been exposed to thelight of the laser. In some embodiments, the upper edge protrudes by atleast 5 μm, or by at least 10 μm, and/or by not more than 20 μm withrespect to areas of the surface to be coated that have not been exposedto the light of the laser.

In one embodiment of the present disclosure, the component is located ina gas atmosphere of noble gas, such as argon or of nitrogen, during itsprocessing. In one embodiment, the component is then in a space throughwhich, for example, a noble gas or nitrogen is passed, or which isgas-tight and into which noble gas or nitrogen has previously beenintroduced. Alternatively or additionally, a protective gas jacket iscreated which surrounds the laser beam during exposure. A protective gasjacket can be created by means of nozzles arranged around the light ofthe laser in such a way that the light beam of the laser impinging onthe component is enveloped by the gas emerging from the nozzles. Theadhesion between the component and the coating can be further improvedin this way.

In one embodiment of the present disclosure, the coating materialcorresponds to the material of the surface to be coated. If the materialof a component has been damaged by external influences, such damage canbe repaired in a particularly stable and reliable manner by a coatingaccording to the present disclosure with the same material.

In one embodiment of the present disclosure, beads of crater-shapeddepressions adjoin each other and/or overlap or intersect. The diameterof each crater-shaped depression then corresponds approximately to thedistance between two centers of two adjacent crater-shaped depressions.

In one embodiment of the present disclosure, the predetermined locationsand thus the crater-shaped depressions are arranged according to arepeating pattern and thus in a planned manner. This can be, forexample, a checkerboard pattern. In the case of a checkerboard pattern,the predetermined locations and thus the crater-shaped depressions arearranged one behind the other and side by side along a straight line,respectively. In this embodiment, the craters are therefore not randomlydistributed.

In one embodiment of the present disclosure, the gas is heated totemperatures of 500° C. to 1200° C. during cold spraying to thus improveadhesion properties. The component to be coated may not be heated beyondthis. In this way, thermally induced physical and/or chemical changes tothe surface to be coated are avoided. Nitrogen can be used as the gas.

In one embodiment of the present disclosure, the laser beam impinges onthe surface of the component at a right angle to create crater-shapeddepressions. However, it is also possible to provide an angle differentfrom 90° in order to roughen the surface of the component suitably bylaser light.

FIG. 1 shows two sectional views of the surface to be coated generatedaccording to the present disclosure, according to the aforementionedfirst part of the substrates, which were determined using a lasermicroscope. Shown is on the one hand a sectional view of exposedlocations in xz direction and on the other hand in yz direction. The twosectional views illustrate that a crater-shaped depression with adiameter of about 0.4 μm was created at an exposed location. The depthwas just under 0.4 μm.

The centers of the essentially circular crater-shaped depressions wereregularly spaced 40 μm apart on the polished surface of the componentaccording to a checkerboard pattern.

FIG. 2 shows an image of the laser microscope on the surface of acomponent to be coated, i.e. a substrate. This image shows a top view ofthe distribution of the crater-shaped depressions 1 according to acheckerboard pattern created by multiple exposures. Between obliquelyopposite round crater-shaped depressions 1 there are unexposed, polishedareas 2. The edges of the craters have on the edge side a solidificationpattern 3 due to the individual exposure, which protrude upwards, i.e.in the z-direction, by up to approx. 15 μm relative to the unexposed,polished areas. The bead-like solidification patterns 3 form upper edgesof the crater-shaped depressions 1. Upper edges 3 of the craters overlapor at least adjoin each other. The depth of the crater-shapeddepressions 1 in relation to the polished base surface is about 20 μm. Atotal depth of about 35 μm has thus been created.

By means of a laser profilometer, roughnesses of surfaces of substratesprocessed according to the present disclosure and roughnesses ofsurfaces to be coated of the other substrates were determined forcomparison purposes. For substrates processed according to the presentdisclosure, i.e. components with a surface according to FIGS. 1 and 2,an average roughness R_(a) (μm)=7.15 and a maximum surface roughnessR_(max) (μm)=48.85 were determined. For components with a surfaceroughened by sandblasting, a mean roughness R_(a) (μm)=2.31 and amaximum surface roughness R_(max) (μm)=19.12 were determined. Forcomponents with a polished surface, a mean roughness R_(a) (μm)=0.02 anda maximum surface roughness R_(max) (μm)=0.44 were determined.

Three substrates each consisting of the IN738 alloy were coated withIN738 powder having an average diameter of 7.89 μm in the commerciallyavailable equipment “CGT-Oerlikon Metco Kinetics 8000” using awater-cooled D-24 De-Laval nozzle, wherein the first three substratescomprised surfaces to be coated treated according to the presentdisclosure, the second three substrates comprised surfaces to be coatedtreated by sandblasting, and the third three substrates comprisedpolished surfaces to be coated. A pressure of 40 bar nitrogen, a gastemperature of 950° C. and a coating distance of 60 mm were selected ascoating parameters. The layer thicknesses produced in this way wereapprox. 400 μm.

These coated substrates were glued between two cylinders for adhesionpeel tests, so that the substrate was glued to one of the cylinders andthe produced coating to the other cylinder. In the case of thesubstrates with the polished surfaces to be coated and with thesandblasted surfaces to be coated, the coating detached from thesubstrate. In the case of the surfaces to be coated treated according tothe present disclosure, one cylinder always detached from the coatedsubstrate. It was thus found that in the case of the surfaces to becoated treated according to the present disclosure, a significantlybetter adhesion between the coating and the substrate was achieved.

FIG. 3 shows a transverse section of a component 4 coated according tothe present disclosure with a layer 5 on the substrate 6 in the regionof a crater-shaped depression 1 after the adhesion peel tests have beenperformed. FIG. 3 illustrates that the crater-shaped depressions 1 havebeen completely filled by the cold spraying and that the adhesion peeltests performed could not change this. No cracks have formed either.

The present disclosure is not limited to the exemplary embodiment. Forexample, it is also possible to prepare the surface of a componentalternatively or additionally in other ways compared to grinding andpolishing, for example by sandblasting or shot peening, in order tosubsequently roughen it by means of a laser. Good adhesion propertiescan also be achieved in this way, especially in comparison with surfacepreparation that comprises only sandblasting for subsequent coating. Toachieve good adhesion properties, another thermal coating process can beused instead of cold spraying, in which the coating material is broughtto a high velocity in order to coat. For example, HVAF (High VelocityAir-Fuel) can be provided instead of cold spraying to achieve goodresults. However, the mentioned alternatives relating to sandblasting,shot peening or HVAF are less preferable. Thus, the task of the presentdisclosure can also be solved by a method for coating a component, inwhich a surface to be coated of the component is roughened by a laserand the roughened surface is coated by a thermal spraying process withhigh kinetic energy such as cold spraying or HVAF.

Compared to literature values, the maximum achievable layer thicknesscould be tripled by the present disclosure.

It is one task of the present disclosure to further develop the coatingof a component. The task may be solved by the present disclosure,including as described in the claims.

The inventors have found that a combination of the two measures“roughening by laser” and “coating by cold spraying” lead to a stableadhesion between the component and the coating layer. It has been foundthat roughening by means of a laser does not adversely modify thematerial of the surface of the component physically or chemically insuch a way as to reduce adhesion between component and layer. Rougheningby means of a laser is therefore advantageous over roughening bysandblasting. It has also been found that cold spraying does notadversely modify the physical and chemical properties of the surfaceeither, so that a very good adhesion between the component and thecoating can be achieved by a combination of the two measures.

A large number of tests were performed in which substrates consisting ofalloy IN738 were used as components and these were coated with apowdered alloy IN738 by cold spraying.

Substrates consisting of the alloy IN738, i.e. components, were firstground and then polished with a 1 μm diamond suspension, i.e. withdiamond grains having an average diameter of 1 μm, on a soft cloth.After cleaning with ethanol in an ultrasonic bath, a first portion ofthe substrates was brought to a laser processing system commerciallyavailable under the name “Laser Marking Trumpf 5020.” This comprises anNd-YAg laser with a laser light wavelength of 1064 nm. The focus of thelaser was set to 100 μm below the substrate surface to be coated. Argonwas used as the atmosphere during exposure to the laser to avoiddetrimental oxidation. In pulsed mode, at full power, corresponding to apeak power of 15 kW, the substrate surface to be coated was ablated at afrequency of 35 kHz and a pulse length of 120 ns. Crater-shapeddepressions with a diameter of 40 μm were thus created on the surface tobe coated. Craters were made one behind the other and side by side,i.e., checkerboard-like, with a spacing of 40 μm. Each individuallocation where a crater was to be created was exposed to the light ofthe laser a total of 12 times. However, a single location was notexposed immediately again, following an exposure. Instead, each of thelocations was exposed for a first time and then, in the same order, fora second time and so on, until all locations had been exposed twelvetimes in this way. Each individual location was therefore allowed tocool down and solidify again after an exposure before being exposedagain. Unwanted oxidation of the surface could thus be avoided in animproved manner.

For comparison tests, a second part of the substrates was not furtherprocessed. The surface to be coated thus remained in the state smoothedby grinding and polishing.

For comparison tests, the surfaces to be coated of substrates consistingof the alloy IN738 were also roughened by sandblasting. These surfacesto be coated had also been previously ground and polished.

1. A method for coating a surface of a component comprising steps of:roughening the surface of the component by a laser in a crater-shapedmanner and coating the roughened surface of the component by a thermalspraying process with high kinetic energy.
 2. The method of claim 1,wherein the surface to be coated is ground and/or polished beforeroughening.
 3. The method of claim 1, wherein the surface to be coatedof the component is repeatedly exposed by laser in pulses atpredetermined locations in such a way that crater-shaped depressions areformed at the predetermined locations.
 4. The method of claim 3, whereintime intervals are provided between the pulses in such a way thatmaterial melted by light of the laser can first solidify again beforethe predetermined location is again exposed by the light of the laser.5. The method of claim 3, wherein each predetermined location is exposedat least 3 times and not more than 25 times by light of the laser. 6.The method of claim 3, wherein, by laser roughening, crater-shapeddepressions with a depth of at least 10 μm and not more than 60 μm areproduced.
 7. The method of claim 6, wherein, by laser roughening,crater-shaped depressions are produced in which the diameter of eachcrater-shaped depression at the upper edge thereof corresponds to aboutthe depth of the crater-shaped depression.
 8. The method of claim 7,wherein the crater-shaped depressions have a diameter of 30 μm to 50 μmat the upper edge and a depth of 30 μm to 50 μm.
 9. The method of claim1, wherein the component is located in a gas atmosphere of noble gas,including one of argon and nitrogen, during at least one of theroughening and the coating.
 10. The method of claim 1, wherein theroughened surface is coated by cold spraying or HVAF.
 11. The method ofclaim 1, wherein the coated surface includes crater-shaped depressionshaving circular diameters and having a depth of at least 20 μm and notmore than 60 μm.
 12. The method of claim 11, wherein an upper edge ofeach crater-shaped depression protrudes in a bead-like manner withrespect to adjacent polished areas of the coated surface.
 13. The methodof claim 12, wherein upper edge of each crater-shaped depressionprotrudes in a bead-like manner with respect to adjacent polished areasof the coated surface by at least 10 μm.
 14. The method of claim 1,wherein the coated surface includes crater-shaped depressions havingcircular diameters and having a depth of not more than 60 μm.
 15. Themethod of claim 3, wherein each predetermined location is exposed notmore than 25 times by light of the laser.
 16. The method of claim 3,wherein, by laser roughening, crater-shaped depressions with a depth ofnot more than 60 μm are produced.
 17. The method of claim 1, wherein thestep of roughening the surface of the component includes impinging thesurface with a laser beam of the laser as a right angle to the surface.18. The method of claim 9, wherein the gas is heated to a temperaturewithin a range of about 500 degrees Celsius to about 1200 degreesCelsius.
 19. The method of claim 3, wherein the predetermined locationsform a repeating pattern on the surface of the component.
 20. The methodof claim 1, wherein the coated surface includes crater-shapeddepressions having circular diameters and having a depth of not morethan 20 percent of the circular diameter.